In recent years, a urea SCR (Selective Catalytic Reduction) system is receiving attention as a technique for cleaning NOx-containing exhaust gas emitted from internal combustion engines such as gasoline engines and diesel engines (where NOx stands for nitrogen oxides). In the urea SCR system, ammonia (NH3) is chemically reacted with nitrogen oxides (NOx) to reduce the nitrogen oxides to nitrogen (N2), and the exhaust gas containing the nitrogen oxides is thereby cleaned.
In the urea SCR system, when the amount of ammonia supplied for the nitrogen oxides is excessively large, unreacted ammonia may be contained in the exhaust gas and emitted to the outside. To reduce the emission of ammonia described above, a multi-gas sensor capable of measuring the concentrations of a plurality of gases including ammonia contained in the exhaust gas is used for the urea SCR system.
In one known multi-gas sensor, an ammonia sensor including a reference electrode, a detection electrode, and a solid electrolyte body sandwiched between these electrodes is attached to an NOx sensor (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2013-221930).